Referring to FIGS. 1 and 1A-1E, a prior-art device for forming a top film sheet 1 from a film web 3 wound on a roll 2 and setting it onto an object to be packaged (not shown) is known from specification EP 0 336 517 B1 and from a prior-art wrapping machine marketed under the trademark “Octopus”. This prior-art device for setting a sheet comprises a machine frame 4 supported on a fixed base. The machine frame 4 comprises vertical upright columns 5, which are outlined in FIG. 1 in dotted broken lines.
The device further comprises a frame 6, which comprises two parallel guide beams 7 disposed at a distance from each other and secured by their ends to each other with transverse bars 8 to form a rectangular horizontal frame structure. The frame 6, guided by the upright columns 5, is arranged to be movable upwards and downwards in the vertical direction.
Such a device is normally used in conjunction with a prior-art wrapping machine, where a top film sheet is first set onto an object to be wrapped, usually a loaded pallet, whereupon the object is wrapped around with film web in a spiralling fashion. For this purpose, a wrapping frame may be attached to the aforesaid frame 6 to guide a film dispenser delivering the film (as in FIG. 2). Alternatively, the film dispenser may be connected to a crank that turns about a vertical axis, as in specification EP 0 336 517 B1.
The device further comprises a holding device fixedly mounted on the guide beams 7 and extending transversely between them. The holding device 9 comprises a pair of holding jaws 10, 11, which are movable relative to each other between a closed position I and an opened position II. In the closed position I (see FIGS. 1A, 1B, 1D and 1E), the holding jaws 10 and 11 are pressed together from opposite sides of the film web to grip the film web between the holding jaws 10 and 11. In the opened position II (see FIG. 1C), the holding jaws 10 and 11 have a first gap 12 between them to allow the film web 3 to move between the holding jaws.
The device further comprises a cutting device 13, which is disposed in the vicinity of the holding device 9 for severing the film web 3.
The device additionally comprises a pulling device 14, which extends transversely between the guide beams 7 and is mounted on the guide beams so as to be movable between a first position A (see FIG. 1B) located near the holding device 9 and a second position B located at a distance from the holding device 9 (see FIGS. 1A, 1C-1E).
As shown in FIG. 1A, the pulling device 14 comprises a pair of pulling jaws 15, 16, which are movable relative to each other between a closed position III and an opened position IV. In the closed position III (see FIGS. 1A, 1C and 1D), the pulling jaws 15, 16 are pressed together from opposite sides of the film web to grip the film web between the pulling jaws. In the opened position IV (see FIGS. 1B and 1F), the pulling jaws 15, 16 have a second gap 17 between them to release the film web 3 from the grip of the pulling jaws.
As can be seen from FIG. 1, the pulling jaws 15 and 16 are claws which have a point-like grip on the film. ‘Point-like grip’ here means a grip over a very small surface area. The claws 15, 16 hold the film web with a point-like grip. The device has several pairs of claws 15, 16 at a distance from each other over the width of the film web. Several pairs of claws 15, 16 are provided to enable the pulling device to pull webs of different widths so that the claws always engage the film by its edges and additionally between the edges. The thinner the film used, the more pairs of claws 15, 16 are needed in the pulling device to distribute the pulling force over the film. For the claws 15, 16 to be able to grip the end of the film web, the support 19 included in the holding device 9 and serving as a counter-element for the cutting blade must be provided with notches 34 corresponding to the claws to receive the claws when the pulling device 14 fetches the end of the film web from the holding device 9.
Referring to FIGS. 1A-1E, the device in FIG. 1 is operated according to the prior-art method in the following steps.
FIG. 1A visualizes that, at the beginning of the cycle, the holding device 9 engages the film web 3 with a first grip near its free end on a first side of the object to be packaged.
As shown in FIG. 1B, the pulling device 14 engages the free end of the film web with a second grip.
As shown in FIG. 1C, the first grip of the holding device 9 is released and, maintaining the second grip of the pulling device 14 on the end of the film web, the free end of the film web is pulled horizontally to a distance from the point of the first grip onto the object to be packaged, to its second side, said distance corresponding to the length of the top film sheet to be formed.
As shown in FIG. 1D, the film web is held fast with a third grip by the holding device 9 on the first side of the object to be packaged.
As shown in FIG. 1E, the film web is severed by means of a cutting device 13 along a cutting line near the third grip to form a top film sheet 1 and the second grip on the end of the film web is released, thus allowing the top film sheet 1 to fall freely onto the object to be packaged. The third grip taken by the holding device 9 constitutes the first grip on the end of the film web for a new sheet forming cycle. To form the next sheet, the steps of FIGS. 1A-1E are performed again.
A problem with the prior-art device and method is that all the pairs of claws 15, 16 must be made to grip the film web with equal force and that a sufficient pressing power must be available to allow the film web to be pulled from the roll 2, which does not necessarily always turn very easily and therefore pulling the film requires the use of force. In the case of thin film, a large number of claw pairs 15, 16 must be available and the claws and notches 34 have to be fitted to the width of the film to be used. In accordance with the positions of the claw pairs, the counter-support 19 has to be provided with many different arrays of notches 34 because, depending on the user's needs, it may sometimes be necessary to use a narrow film and sometimes a wide film. These have to be fitted in such a way that the device will work properly. The outermost notch and claw pair have to be very close to the side edge of the film web. Such individual fitting work is time-consuming and difficult. Due to the notches 34 in the cutting counter-support 19, problems also appear when the film is being severed, because in the area of the notch 34 the film web may not be severed completely because in the region of the notch the film can withdraw before the cutting blade. In the prior-art device, problems occur especially when the film is very thin, e.g. of the order of 15 μm.